After defeating Mulcarn, Kylorin searched the collapsing stronghold (which Auric would later name Letum Frigus) to find the Heart of Sucellus, the final piece that Nantosuelta needed in order to reassemble her beloved an allow his resurrection. The Amurite flag (which is used anachronistically in AoI, much like the name Letum Frigus) is a depiction of Kykorin emerging from one of the caverns leading from the fortress to find the sun shining brightly and starting to melt away the ice like it had never done during Mulcarn's reign on earth. This was however not yet the end of the Age of Ice. After his victory over the God of Winter, Kylorin trekked across Erebus to find the Tomb of Sucellus, handed over the Heart to the priests of Nantosuelta, and witnessed the the return of Sucellus as God of Life. That was the event that the scholars of Erebus consider the end of the Age of Ice and the start of the Age of Rebirth. It was also the last confirmed sighting of Kylorin.

He is very probably still alive, but has grown weary of the burdens of power and prestige. He wants only to live peacefully and anonymously with his beloved wife, who due to his ancient pact with Ceridwen will forever be reincarnated in Erebus rather than finding rest of Arawn's vault.

(I don't have much of a basis for this, but I personally like to think that the first identity Kylorin took on after his mysterious disappearance was that of Jonas Ulvin, as he had discovered his beloved reincarnated in the form of a girl named Harna living in the small Illian village of Brigdarrow. As he was living among his recent enemies, he had to be extremely careful to hide his identity and could not even train his son Auric to control his innate and enormous magical potential. When he found Harna killed by the goblins that sacked the village, he forgot about his son and left to find her next incarnation. This left Auric without guardian or guide as he struggled to comprehend his emerging power. A couple nights of lessons with Talia Gossam and reading texts from library of Barathrum were woefully inadequate to teach a boy with such potential how to resist the ungoverned precept of winter dominating his will and driving him into utter madness as it sought a new master.)

When the Ascension ritual is completed, in addition to creating the Auric Ascended it also creates The Godslayer and triggers an event to reveal it. The text of this event describes an old man coming to you with a map showing the location of the blade. This man is almost certainly Kylorin himself. (However, in the scenarios this event did not happen, The Godslayer was not found, and The Netherblade was the only available weapon capable of slaying Auric Ascended.)

Suppose the pact made between Kylorin and Ceridwen was discovered by Laroth, and he sent an assassin after her with the Netherblade - what would happen to her? Would she go to Laroth's domain (technically it is in Arawn's vault)? Would the pact override the blade's powers?

For that matter, what would happen to Kylorin, since he cannot die? I wonder if Laroth has considered this as a way of tremendously increasing his power?

(It's also an interesting thought if in fact Magister's theory about Kylorin, his wife, and Auric is actually true. It might be a way for Kylorin and his wife, the whole family really, to be together permanently.)

I assume you mean what would happen if Eve/Epona were attacked with the Natherblade? Your question made it sound like you wanted to know what would happen if Ceridwen herself was struck with the blade. It is hard to imagine the goddess that can move effortlessly between more world than any other being even knows exist being bound to a single plane by an artifact fashioned by Laroth. I believe that Kael has stated that Os-Gabella would probably be pulled entirely, body and soul, into the Netherworld, but would retain the power to leave again at will. Surely the Goddess of the Stars could do the same.

Assuming you meant Eve, I would guess that the power of Ceridwen causing her soul to forever return and be incarnated into a new infant girl would probably be stronger than the power of Laroth. It might be interesting to test it though.

Kael has explicitly stated that Kylorin can die. His bargain with Ceridwen prevents him from further aging and makes it possible to live indefinitely, but does not grant him anything close to the immortality of Os-Gabella. True immortality like hers can only be bestowed (or revoked) by The One or by The Twenty One all acting in unison, not by a single rouge goddess.